1. Field of the Invention
The present invention relates to an ion current detector which detects an ion, for example, generated by combustion in an internal combustion engine.
2. Description of the Related Art
In recent years, the problems of environmental conservation and fossil fuel depletion have been raised and it becomes an urgent need to deal with these problems also in the automotive industry. As an example dealing with this, there is ultra-lean combustion (stratified lean combustion) operation of an engine which utilizes a stratified fuel-air mixture. In the stratified lean combustion, the concentration of the fuel-air mixture may vary; and therefore, in the case of combustion in a locally rich state, soot is generated and carbon adheres to an ignition plug. Accordingly, a leak caused by a smolder is generated.
Under conditions where the smolder leak is generated, there is a high possibility that fails to ignite the fuel-air mixture; and from this standpoint, a technique for monitoring whether or not intended combustion is obtained is desired. As one of such methods, methods which detect an ion generated by the combustion as a current and monitor the combustion state from generation conditions of the ion current are proposed in large numbers.
As a device for detecting such ion current, for example, in an internal combustion engine ignition device disclosed in Patent Document 1, an ignition circuit of an ignition plug is configured so as to perform multiple discharge by so-called a capacitor discharge ignition (CDI) system which includes an ignition coil in which an ignition plug is connected to a secondary side winding L2, a capacitor which is connected in series to a primary side winding L1 of the ignition coil, a boosting circuit which charges the capacitor by boosting a voltage of a battery BT, a thyristor which intermits a closed-loop formed by the capacitor and the primary side winding L1, and a timer circuit which repeatedly outputs a trigger signal TG that makes the thyristor turn on at a constant discharging interval during the time defined by an ignition signal IG that is set depending on an operating state. The capacitor is charged by utilizing a secondary current flowing through the secondary side winding of the ignition coil and a voltage (bias voltage) across the ends of the charged capacitor is applied to a central electrode of the ignition plug via the secondary side winding of the ignition coil; and accordingly, an ion current that flows by an ion generated by combustion of a fuel-air mixture is detected to get a grasp of the combustion state.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2000-45924
However, in the internal combustion engine ignition device of Patent Document 1, a method for charging a capacitor by utilizing the secondary current is adopted; and there are concerns that the capacitor cannot be sufficiently charged to a necessary voltage when capacitance of the capacitor is set to be large, and energy is consumed for charging the capacitor and therefore energy for spark discharge is reduced to deteriorate ignition properties to the fuel-air mixture. Therefore, a capacitor whose capacitance is relatively small is used for an actual device in consideration of such circumstances.
But, in the case where the smolder leak of the ignition plug is generated, the smolder leak being apt to generate in the stratified lean combustion, problems exist in that energy stored in the capacitor flows out as a leakage current from a path of the leak caused by the smolder and therefore a bias voltage for detecting an ion current is reduced to deteriorate the detection properties of the ion current, and the proportion of the amount of energy flow with respect to the capacitance of the capacitor increases and therefore the leakage current associated with the lowering of the bias voltage has a waveform shape that simply attenuates during one ignition cycle and it becomes difficult to discriminate between the ion current and the leakage current.